FIG. 1a depicts a conceptual view of a frequency selective surface 20 without varactor diodes (which varactor diodes or other variable capacitance devices can be used to realize an electrically steerable surface wave antenna—see FIG. 2a). The surface 20 of FIG. 1a comprises a plane of periodic metal patches 22 separated from a ground plane 26 by a dielectric layer 21 (not shown in FIG. 1b, but see, for example, FIGS. 2a and 2b). An antenna (not shown) is typically mounted directly on the frequency selective surface 20. See, e.g., U.S. Pat. No. 7,068,234 issued Jun. 27, 2006. The thickness of the dielectric layer 26 can be less than 0.1 of a wavelength of operational frequency of the non-shown antenna. This surface 20 supports a fundamental TM surface wave as shown in its dispersion diagram (frequency vs. propagation constant) of FIG. 1b. The surface impedance of any TM surface wave structure can be calculated by using:ZTM=jZo{(β/ko)2−1}
where Zo is characteristic impedance of free space, ko is the free space wavenumber and β is the propagation constant of the mode.
FIG. 1a depicts the basic structure that supports a fundamental TM surface wave mode. A dielectric substrate 21 (see FIGS. 2a and 2b, not shown in FIG. 1a for ease of illustration) between the plane of metallic patches 22 and the ground plane 26 provides structural support and is also a parameter that determines the dispersion of the structure. This structure can be made using printed circuit board technology, with a 2-D array of metallic patches 26 formed on one major surface of the printed circuit board and a metallic ground plane 26 formed on an opposing major surface of the printed circuits board, with the dielectric of the printed circuit board providing structural support. The equivalent circuit model of the structure is superimposed over the physical elements of FIG. 1a: a series inductance (LR) is due to current flow on the patch 22, a shunt capacitance (CR) is due to voltage potential from patch 22 to ground plane 26, and a series capacitance (CL) is due to fringing fields between the gaps between the patches 22. The dispersion diagram of FIG. 1b shows that a fundamental TM forward wave mode (since the slope is positive) is supported.
In order to control the dispersion and thus the surface impedance at a fixed frequency of the surface shown in FIG. 1a, the gap capacitance (between neighboring metal patches 22) can be electrically controlled by the use of varactor diodes 30. The varactor diodes 30 are disposed in the gap between each patch 22 and are connected to neighboring patches 22 as shown in FIG. 2a. However, since a DC bias is required in order to control the capacitance of the varactor diodes 30, the structure of FIG. 1a has been modified to include not only varactor diodes 30 but also a biasing network supplying biasing voltages V1, V2, . . . Vn. FIG. 2b shows a cross-sectional view of the structure of FIG. 2a with varactor diodes and the aforementioned biasing network; every other patch is connected directly to the ground plane 26 by conductive grounding vias 24 and the remaining patches are connected to the biasing voltage network by conductive bias vias 28. See, for example, U.S. Pat. Nos. 6,538,621 and 7,068,234 for additional information.
However, the addition of the bias vias 28 penetrating the ground plane 26 at penetrations 32 introduces a shunt inductance to the equivalent circuit model superimposed in FIG. 1a. FIG. 3a depicts a model similar to that of FIG. 1a, but showing the effect of introducing the bias network of FIGS. 2a and 2b by a shunt inductance LL. As shown by FIG. 3b, TM backward wave is supported when a series capacitance and a shunt inductance are present, the latter of which is contributed by the bias via 28. The backward wave decreases the frequency/impedance range of the surface wave structure since one can couple to only a forward wave or to a backward wave at a given frequency.
It would be desirable to allow for control of the dispersion and thus the surface impedance of the frequency selective surface of FIG. 1a by using variable capacitors (such as, for example, varactor diodes) as taught by Sievenpiper (see, for example, U.S. Pat. No. 7,068,234) and in FIGS. 2a and 2b hereof, but without the introduction of a backward wave.